Loading Rate Dependence of Unstable Behaviors of Thin-Shell Structured Columns under Axial Compression

Abstract

The thin-shell structured columns that are used extensively in the vehicle body need to satisfy both strength requirements for crash safety and demands for weight reductions for environmental friendliness. This study focused on the loading rate dependence of unstable behavior, especially the maximum value of reaction force, which is generated in the thin-shell structured columns as a result of axial compression due to a frontal crash. The mechanism generating the reaction force was made clear through a comparison with classical Euler buckling and von Karman's effective width expression. It was observed that a square cross section displays markedly large loading rate dependence, which can be approximated well by considering the effect of inertial force in the high loading rate region and by von Karman's effective width solution in the low loading rate region. Especially in the latter region, Euler buckling accompanying the out-of-plane instability without any reaction descent preceded the global instability which can be described by Karman's solution. When the cross section shape is changed to hexagon and octagon, it was found that the loading rate dependence, that is, the descent of maximum reaction force from that in the uniform plastic deformation of whole cross section became small. And for circular pipe as a limiting case of polygon, the smallest loading rate dependence was observed.